1. Field of the Invention
The present invention generally concerns optical traps and tweezers for, and optical trapping and tweezing of, small and microscopic objects.
The present invention particularly concerns (i) a laser and laser arrays for optical traps and tweezers, (ii) the structure and operation of optical traps and tweezers based on the laser and laser arrays, and (iii) the properties of output light from this (these) laser(s) and laser arrays.
2. Description of the Prior Art
Optical trapping generally enables transport of fine particles based on radiation pressure. Optical traps, or optical tweezers, act like the “tractor beams” of the fictional starship Enterprise on a microscopic scale. In 1985, A. Ashkin trapped small particles with focused laser beam. See A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles”, Optical letters, vol. 11, no. 5, 1986. See also A. Ashkin, Science 210, 1081, 1980. This seminal work by Arthur Ashkin, Joe Dziedzic, John Bjorkholm, and Steven Chu at Bell Laboratories (now Lucent Technologies) demonstrated how to pick up and move tiny latex spheres using nothing more than a microscope lens and a low-power laser.
This was followed by the demonstration of live biological cell trapping. See A. Ashkin and J. M. Dziedzic, “Optical trapping and manipulation of viruses and bacteria”, Science, vol. 235, 1987. See also A. Ashkin, J. M. Dziedzic and T. Yamane, “Optical trapping and manipulation of single cells using infrared laser beams”, Nature, vol. 330, 1987.
Still more recently, Chu, a recent Nobel Prize winner, has used tightly focused beams of light to capture and manipulate strands of DNA.
For biological applications, near infrared lasers are used to prevent damage due to light absorption. Nd: YAG, Nd: YLF, Ti: Sapphire or diode lasers are the most commonly used light sources for optical trapping applications. Besides single-beam optical traps, multi-beam tweezers were developed to manipulate small size objects (2 mm). The most common methods in building multi-beam tweezers can be listed as follows:
First, a single beam can be split with a beamsplitter and recombined with refractive optics. See K. Sasaki et al., Optical Letters 16, 1463, 1991. This technique is limited by the number of trapping beams.
A second method of optical trapping, or tweezing, produces time-averaged extended trapping patterns made by the fast scanning of a single trapping beam. This method, producing simple interference fringe patterns, is limited.
A third method of optical trapping, or tweezing, is based on split and steered light from a single beam with diffractive optical elements. Computer generated holographic techniques can be used to create customized particle arrangements with a single beam. See K.
Sasaki et al., Optical Letters 16, 1463, 1991. See also L. P. Faucheux and A. J. Libchaber, Phys. Rev. E 49, 5158, 1994. Only particles having a low index of retraction can be trapped with this method. Moreover, optical waveforms generated by holograms are notoriously non-uniform, and do now have any adjustability.
Finally, addressable liquid crystal phase shifting arrays permit dynamically active tweezing.
The existing methods do not, to the best knowledge of the inventors, use a Vertical Cavity Surface Emitting Laser (VCSEL) as the source of a laser beam, let alone many such VCSELs.
The existing methods do not, to the best knowledge of the inventors, permit multiple objects to be manipulated in parallel at the same time. Neither do they permit multiple “trapping” or “tweezing” optical beams to be focused onto a single, potentially quite large, object at a single time in order to exert more optical force on the object. The present invention will be seen to permit either, and both.
The laser beam(s) used in existing optical trapping and tweezing methods have, to the best knowledge of the inventors, energy distributions—meaning the distribution of illumination energy across, and perpendicular to, a laser beam—the that are uncontrolled, and that are most commonly Hermite-Gaussian. A laser so producing a laser beam of Hermite-Gaussian energy distribution is spoken of as operating in Hermite-Gaussian mode, which mode is the default operating mode of a laser. The default production of laser beams of Hermite-Gaussian energy distribution is perhaps understandable when it is understood that it has not heretofore been understood how to create, save possibly by the use of a holographic element (which holographic element would be at least cumbersome and more likely completely unsuitable in optical tweezers), a laser beam having a superior Laguerre-Gaussian energy distribution. The present invention will be seen to prefer the use of a laser beam having the superior Laguerre-Gaussian energy distribution in optical trapping and tweezing. The present specification disclosure will, by incorporating by reference a co-pending patent assigned to the same assignee as is the present invention, show how such a Laguerre-Gaussian energy distribution laser beam may reliably be realized, including as is produced by a VCSEL.
Existing optical trapping and tweezing methods neither contemplate nor offer much controllability in the power of the laser beam, it being deemed sufficient that the beam remains adequately intense so as to effect the desired spatial manipulations of the trapped or tweezed object at the desired rates. However, should optical trapping and tweezing be contemplated to transpire in parallel upon a one- or two-dimensional array of manipulated objects by use of a corresponding array of laser beams (both of which one and two-dimensional arrays will be seem to contemplated by the present invention) then the laser beams would desirably be uniform across the array, including by potential to independently adjust the intensity of each laser beam if warranted. The present invention will show that some array manipulations do so warrant adjustment of the arrayed manipulating laser beams, and will how this may be realized in a broad and substantial manner.
Finally, most existing optical tweezing apparatus are extended in size, and are not compact, due to the required bulky laser sources. The present invention will be seen to be opposite, and to be compact in size.
The existing optical tweezing methods and apparatus are also, commonly, constrained in (i) the types of applications which can be realized, and/or (ii) the nature and range of the (small) objects subject to manipulation, by a single method, or a single apparatus. This is, of course, opposite to common macroscopic mechanical tweezers, or pliers, or the like which usefully fit a broad range of work pieces. The present invention will be seen to address this issue, and to concern a new method, and apparatus, capable of realizing ubiquitous, and versatile, optical trapping and tweezing functions. See M. Ozkan, M. M. Wang and S. Esener “Pick and Place of organic and inorganic devices with VCSEL driven optical micro-beams”, HOTC conference proceedings, Santa Barbara, July 2000.